The app connects users with a trio of tools on the main www.usps.com website. Alternatively, they can point a mobile browser to the service's mobile site, m.usps.com. The iPhone app currently is the only one of the three that lets users calculate shipping costs.

Over the past two years, the USPS has released similar apps for the iPhone and for Android. Together, they've been downloaded by 1.2 million users, who generate 4 million visits each month to the main website, according to a USPS statement.